Diesel engines are generally slow and difficult to start at low temperatures. To overcome this they are equipped with a glow plug in an auxiliary combustion chamber. The glow plug is heated by an electic current until it is red-hot at which time an air-fuel mixture is introduced into the combustion chamber. The glow plug raises the temperature of the air-fuel mixture sufficiently to permit ignition and engine staring.
The glow plug must withstand the high temperatures of ignition. Also, it must be durable because of the common use of a glow plug as an after-glow plug that stabilizes combustion after the engine has been started.
In order to endure frequent rapid heating and use as an after-glow plug, conventional glow plugs have used a ceramic heater as a heating element. The ceramic heater is prepared by shaping and sintering a silicon nitride (Si.sub.3 N.sub.4) based ceramic powder having a wire heater embedded therein. The wire is made from a metal with a high melting point such as tungsten.
The essential parts of a conventional ceramic glow plug are shown in partial cross section in FIG. 2. A ceramic heater 2 (hereinafter referred to simply as "heater") having a coil of heating wire 1 embedded therein is brazed to the inner surface of a bore in a metal tube 3. A heating tip 2--1 is left exposed. A lead wire 1--1 is connected to one end of the coil 1 and at its other end is electrically connected to the metal tube 3. The metal tube 3 is brazed to the lower end of an axial bore of a metal holder 4. The metal holder 4 is connected to a negative electrode. A lead wire 1--2 is connected to the other end of the coil 1, extends from the rear of the heater 2, and is connected to a positive electrode terminal (not shown). The lead wire 1--2 is insulated from the metal holder 4 at the rear end of the glow plug.
In the heater 2 of the conventional ceramic glow plug shown in FIG. 2, sintering aids, e.g. Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 and M.sub.g O, incorporated in the silicon nitride sinter react with the latter to form glassy material between Si.sub.3 N.sub.4 particles. In the prior art, the sintered surface of the heater is not given any finishing treatment or is sometimes polished. The tip of the heater is inserted into the combustion chamber of the engine and is exposed directly to a high-pressure and high-speed gas stream in a transient atmosphere where a temperature of 1,000.degree. C. or higher is prevalent. Simultaneous oxidation and reduction occur under these conditions.
The glassy material between silicon nitride particles is eroded, corroded, or otherwise broken down to cause gradual thinning of the heater. As a result, heat condition from the wire heater 1 buried in the ceramic matrix varies greatly and not only does it become impossible to attain the desired high temperature but cracks may develop in the ceramic matrix. Thus, until now glow plugs have not performed in a completely satisfactory manner.